Sample records for thorianite
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Grimaldi, F.S.
1957-01-01
This paper presents a selective iodate separation of thorium from nitric acid medium containing d-tartaric acid and hydrogen peroxide. The catalytic decomposition of hydrogen peroxide is prevented by the use of 8quinolinol. A few micrograms of thorium are separated sufficiently clean from 30 mg. of such oxides as cerium, zirconium, titanium, niobium, tantalum, scandium, or iron with one iodate precipitation to allow an accurate determination of thorium with the thoronmesotartaric acid spectrophotometric method. The method is successful for the determination of 0.001% or more of thorium dioxide in silicate rocks and for 0.01% or more in black sand, monazite, thorite, thorianite, eschynite, euxenite, and zircon.
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Evolution of spent nuclear fuel in dry storage conditions for millennia and beyond
NASA Astrophysics Data System (ADS)
Wiss, Thierry; Hiernaut, Jean-Pol; Roudil, Danièle; Colle, Jean-Yves; Maugeri, Emilio; Talip, Zeynep; Janssen, Arne; Rondinella, Vincenzo; Konings, Rudy J. M.; Matzke, Hans-Joachim; Weber, William J.
2014-08-01
Significant amounts of spent uranium dioxide nuclear fuel are accumulating worldwide from decades of commercial nuclear power production. While such spent fuel is intended to be reprocessed or disposed in geologic repositories, out-of-reactor radiation damage from alpha decay can be detrimental to its structural stability. Here we report on an experimental study in which radiation damage in plutonium dioxide, uranium dioxide samples doped with short-lived alpha-emitters and urano-thorianite minerals have been characterized by XRD, transmission electron microscopy, thermal desorption spectrometry and hardness measurements to assess the long-term stability of spent nuclear fuel to substantial alpha-decay doses. Defect accumulation is predicted to result in swelling of the atomic structure and decrease in fracture toughness; whereas, the accumulation of helium will produce bubbles that result in much larger gaseous-induced swelling that substantially increases the stresses in the constrained spent fuel. Based on these results, the radiation-ageing of highly-aged spent nuclear fuel over more than 10,000 years is predicted.
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Preliminary summary review of thorium-bearing mineral occurrences in Alaska
Bates, Robert G.; Wedow, Helmuth
1952-01-01
Thorium-bearing minerals are known at 47 localities in Alaska. At these localities the thorium occurs as a major constituent or in minor amounts as an impurity in one or more of the following 12 minerals: allanite, columbite, ellsworthite, eschynite, gummite, monazite, orangite, parisite, thorianite, thorite, xenotime, and zircon. In addition other minerals, such as biotite and sphene, are radioactive and may contain thorium. Several unidentified columbate minerals with uranium or thorium and uranium as major constituents have been recognized at some localities. The distribution, by type of deposit, of the 57 thorium occurrences is as follows: lode - 3, lode and placer - 1, granitic rock - 3, granitic rock and related placer - 14, and placer - 26. Of the four lode occurrences only the radioactive veins at Salmon Bay in southeastern Alaska and the contact metamorphic deposit in the Nixon Fork area of central Alaska warrant further consideration, although insufficient data are available to determine whether these two deposits have commercial possibilities. The remaining occurrences of thorium-bearing minerals in Alaska are limited to placer deposits and disseminations of accessory minerals in granitic rocks. In most of these occurrences the thorium-bearing minerals occur in only trace amounts and consequently warrent little further consideration. More data are needed to determine the possibilities of byproduct recovery of thorium-bearing minerals from several of the gold and tin placers.
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Major, trace and REE geochemistry of recent sediments from lower Catumbela River (Angola)
NASA Astrophysics Data System (ADS)
Vinha, Manuela; Silva, M. G.; Cabral Pinto, Marina M. S.; Carvalho, Paula Cristina S.
2016-03-01
The mineralogy, texture, major, trace and rare earth elements, from recent sediment samples collected in the lower Catumbela River, were analysed in this study to characterize and discuss the factors controlling its geochemistry and provide data that can be used as tracers of Catumbela River inputs to the Angolan continental shelf. The sediments are mainly sands and silty-sands, but sandy-silt also occurs and the mineralogy is composed of quartz, feldspar, phyllosilicates, magnetite, ilmenite and also carbonates when the river crosses limestones and marls in the downstream sector. The hydraulic sorting originates magnetite-ilmenite and REE-enriched minerals placers. The mineralogy of the sediments is controlled by the source rocks and the degree of chemical weathering is lower than erosion. The texture is mainly controlled by location. There is enrichment in all the analysed trace elements in the fine grained, clay minerals and Fe-oxy-hydroxides rich sediments, compared to the coarse grained and quartz plus feldspar rich ones. The coarse grained sediments (without the placers) are impoverished in ΣREE when compared with UCC and NASC compositions, while the fine grained sediments have ΣREE contents similar to UCC and NASC. The placers have ΣREE contents up to 959.59 mg/kg. The source composition is the dominant factor controlling the REE geochemistry of the analysed sediments as there is no difference in the (La/Yb)N, (La/Sm)N and (Gd/Yb)N ratios in coarse and fine grained sediments. The sorting of magnetite, ilmenite, zircon, throrite, thorianite, rutile and titanite explain the HREE/LREE enriched patterns of the coarse grained sediments.
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Fedele, Luca; Tarzia, Maurizio; Belkin, Harvey E.; De Vivo, Benedetto; Lima, Annamaria; Lowenstern, Jacob
2007-01-01
The Breccia Museo, a pyroclastic flow that crops out in the Campi Flegrei volcanic complex (Naples, Italy), contains alkali-syenite (trachyte) nodules with enrichment in Cl and incompatible elements (e.g., U, Zr, Th, and rare-earth elements). Zircon was dated at ≈52 ka, by U-Th isotope systematics using a SHRIMP. Scanning electron microscope and electron microprobe analysis of the constituent phases have documented the mineralogical and textural evolution of the nodules of feldspar and mafic accumulations on the magma chamber margins. Detailed electron microprobe data are given for alkali and plagioclase feldspar, salite to ferrosalite clinopyroxene, pargasite, ferrogargasite, magnesio-hastingsite hornblende amphibole, biotite mica, Cl-rich scapolite, and a member (probable davyne-type) of the cancrinite group. Detailed whole rock, major and minor element data are also presented for selected nodules. A wide variety of common and uncommon accessory minerals were identified such as zircon, baddeleyite, zirconolite, pollucite, sodalite, titanite, monazite, cheralite, apatite, titanomagnetite and its alteration products, scheelite, ferberite, uraninite/thorianite, uranpyrochlore, thorite, pyrite, chalcopyrite, and galena. Scanning electron microscope analysis of opened fluid inclusions identified halite, sylvite, anhydrite, tungstates, carbonates, silicates, sulfides, and phosphates; most are probably daughter minerals. Microthermometric determinations on secondary fluid inclusions hosted by alkali feldspar define a temperature regime dominated by hypersaline aqueous fluids. Fluid-inclusion temperature data and mineral-pair geothermometers for coexisting feldspars and hornblende and plagioclase were used to construct a pressure-temperature scenario for the development and evolution of the nodules. We have compared the environment of porphyry copper formation and the petrogenetic environment constructed for the studied nodules. The suite of ore minerals observed in the nodules supports a potential for mineralization, which is similar to that observed in the alkaline volcanic systems of southern Italy (Pantelleria, Pontine Archipelago, Mt. Somma-Vesuvius).
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NASA Astrophysics Data System (ADS)
Sotnikova, Irina; Vladykin, Nikolai
2015-04-01
Burpalinsky rare metal alkaline massif in the Northern Baikal folded zone in southern margin of Siberian Platform, is a of intrusion central type, created 287 Ma covering area of about 250 km2. It is composed of nepheline syenites and pulaskites grading to quartz syenites in the contacts. Veines and dykes are represented by shonkinites, sodalite syenite, leucocratic granophyres, alkali granites and numerous rare metal alkaline syenite pegmatites and two dykes of carbonatites. All rocks except for granites are cut by a large apatite-fluorite dyke rocks with mica and magnetite, which in turn is cut by alaskite granites dyke. The massif has been studied by A.M. Portnov, A.A. Ganzeev et al. (1992) Burpalinsky massif is highly enriched with trace elements, which are concentrated in pegmatite dykes. About 70 rare-metal minerals we found in massif. Zr-silicates: zircon, eudialyte, lovenite, Ti-lovenite, velerite, burpalite, seidozerite, Ca- seidozerite, Rosenbuschite, vlasovite, katapleite, Ca-katapleite, elpidite. Ti- minerals:- sphene, astrophyllite, ramsaite, Mn-neptunite bafertisite, chevkinite, Mn-ilmenite, pirofanite, Sr-perrerit, landauite, rutile, anatase, brookite; TR- minerals - loparite, metaloparite, britolite, rinkolite, melanocerite, bastnesite, parisite, ankilite, monazite, fluocerite, TR-apatite; Nb- minerals - pyrochlore, loparite. Other rare minerals leucophanite, hambergite, pyrochlore, betafite, torite, thorianite, tayniolite, brewsterite, cryolite and others. We have proposed a new scheme massif: shonkinites - nepheline syenites - alkaline syenite - quartz syenites - veined rocks: mariupolites, rare-metal pegmatites, apatite, fluorite rock alyaskite and alkaline granites and carbonatites (Sotnikova, 2009). Apatite-fluorite rocks are found in the central part of massif. This is a large vein body of 2 km length and a 20 m width cutting prevailing pulaskites. Previously, these rocks were regarded as hydrothermal low-temperature phase. New geological and thermobarometric evidence suggests that apatite-fluorite rocks were formed from the residual fluid-melt, separated after crystallization of rare-metal pegmatites. Petrochemical and geochemical data Burpalinsky are in accord of general trend of crystal differentiation of alkaline magma containing small concentrations of CO2 and higher P2O5 and F, which accumulated significantly separated from the pegmatite melts. In some pegmatites fluorite with rare-metal minerals (flyuocerit etc) are separating in schlieren. Apatite-fluorite rocks are cut by leucogranite dyke, having genetic connection with rare-metal pegmatites. Late granitic phases has its own association of rare-metal minerals described by A.A. Ganzeev (1972). Thermobarometric geochemical study of apatite-fluorite rocks Burpala massif found a large number of primary fluid inclusions (15-50 micrometers). Thermal and cryometric research of 60 individual fluid inclusions in fluorite showed the domination of Na, Ca, Mg chlorides and high temperatures salt inclusions in fluorites (above 550C) and melt inclusions in apatites (800C). Apatite-fluorite rocks in massif are similar to foskorites in carbonatite complexes, with similar high Ca content, but instead fluorite, together with other "foskoritovymi" minerals - apatite, magnetite, mica, and pyroxene were formed instead for calcite. Isotopic studies (Sr-Nd) indicate the mantle source of primary magma Burpala massif close to EM-2, which is characteristic of alkaline intrusions in the folded belts (Vladykin 2009). RBRF grant 14-45-04057
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The Distribution of Heat-Producing Radioactive Elements in the Deep Earth
NASA Astrophysics Data System (ADS)
Chidester, Bethany A.
The Earth is a heat engine, where large differences in temperature between the interior and the surface drive large-scale movement that manifests as plate tectonics and the geomagnetic field that protects us from the Sun's harmful charged particles. Decay of the long-lived radioactive elements U, Th, and K is expected to contribute as much as 45% of the current heat production in the Earth, and that heat production was five times higher early in Earth's history. It is unclear how this heat source affects the thermal and dynamic evolution of the Earth's core and mantle and how that contribution has changed over geologic time. This dissertation addresses this problem in several different ways. This work represents the first high-pressure, high-temperature metal-silicate partitioning experiments for U, Th, and K in the laser-heated diamond anvil cell at conditions relevant to core formation. A chemical model is developed using parameterization of these partitioning data to constrain the concentrations of each of these elements in the core. Using a numerical calculation, it is then determined how that radioactive heat would contribute to the core's energy and entropy budget through time. One finds that, despite its strong lithophile nature at the surface, U partitions significantly into the metallic phase at increasing temperatures. This may be due to a decrease in U valence from 4+ to 2+ in high-pressure silicate melts, which our data supports. However, K and Th do not exhibit a similar change in behavior at these conditions, and this may drive fractionation between U and Th in the deep mantle. At the most extreme conditions of core formation, enough U could exist in the core to produce up to 4.4 TW of heat 4.5 billion years ago. Potassium could produce much less heat than U early on (< 1 TW), and due to its short half-life, would have decayed away much faster. While this energy source is significantly greater than was previously thought to be possible, it is likely not enough to explain the presence of the geomagnetic field early in Earth's history. I have also completed a synchrotron-based study to determine the phase behavior and equations of state of UO2 and ThO2. ThO 2 undergoes a phase transition from the fluorite- type structure (thorianite) that is stable at ambient conditions to the previously identified cotunnite-type structure around 19 GPa and 1500 K. It remains in the cotunnite-type phase up to 60 GPa and 2500 K. UO2 undergoes several solid phase transitions at high pressure. The fluorite-type (uraninite) to cotunnite-type transition occurs around 20 GPa above 1100 K. At around 35 GPa, a new phase emerges; this phase has been indexed to a tetragonal crystal structure. Finally, at 80 GPa and above, UO2 undergoes another phase transition or dissociates into two separate oxides. This understanding of the phase behavior of the simplest actinide-bearing minerals provides insight into the mineralogical hosts for these radioactive elements, as well as other large cations, in the Earth's deep mantle.
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Wedow, Helmuth
1956-01-01
In the period 1945-1954 over 100 investigations for radioactive source materials were made in Alaska. The nature of these investigations ranged from field examinations of individual prospects or the laboratory analysis of significantly radioactive samples submitted by prospectors to reconnaissance studies of large districts. In this period no deposits of uranium or thorium that would warrant commercial exploitation were discovered. The investigations, however, disclosed that radioactive materials occur in widely scattered areas of Alaska and in widely diverse environments. Many igneous rocks throughout Alaska are weakly radioactive because of uranium- and thorium-bearing accessory minerals, such as allanite, apatite, monazite, sphene, xenotime, and zircon; more rarely the radioactivity of these rocks is due to thorianite or thorite and their uranoan varieties. The felsic rocks, for example, granites and syenites, are generally more radioactive than the mafic igneous rocks. Pegmatites, locally, have also proved to be radioactive, but they have little commercial significance. No primary uranium oxide minerals have been found yet in Alaskan vein deposits, except, perhaps, for a mineral tentatively identified as pitchblende in the Hyder district of southeastern Alaska. However, certain occurrences of secondary uranium minerals, chiefly those of the uranite group, on the Seward Peninsula, in the Russian Mountains, and in the vicinity of Kodiak suggest that pitchblende-type ores may occur at depth beneath zones of alteration. Thorite-bearing veins have been discovered on Prince of Wales Island in southeastern Alaska. Although no deposits or carnotite-type minerals have been found in Alaska, several samples containing such minerals have been submitted by Alaskan prospectors. Efforts to locate the deposits from which these minerals were obtained have been unsuccessful, but review of available geologic data suggests that several Alaskan areas are potentially favorable for carnotite-type deposits. The chief of these areas is the Alaska Peninsula-Cook Inlet area which encompasses most of the reported occurrences of the prospectors' carnotite-type samples. Alaska is also potentially favorable for the occurrence of large bodies of the very low-grade uraniferous sedimentary rocks, such as phosphorites and black shales. This type of deposit, however, has not received much study because of the emphasis on the search for bonanza-type high-grade ores. Uraniferous phosphorites similar to those of Idaho, Montana, and Wyoming occur in northern Alaska on the north flank of the Brooks Range; black shales comparable to the uraniferous shales of the Chattanooga formation of southeastern United States have been noted along the Yukon River near the international boundary. Placer deposits in Alaska have some small potential for the production of the radioactive elements as byproducts of gold- and tin-placer mining. the placer area believed to have the relatively greatest potential in Alaska lies in the Kahiltna River valley where concentrates are known to contain such commercial minerals as ilmenite, cassiterite, platinum, and gold in addition to uranothorianite and monazite. The possibilities of the natural fluids--water and petroleum--have not yet been tested in Alaska to any great extent. Studies of fluids are in progress to determine whether they may be used to discover and define areas potentially favorable for the occurrence of uraniferous lodes.
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NASA Astrophysics Data System (ADS)
Schoneveld, Louise; Spandler, Carl; Hussey, Kelvin
2015-08-01
The Nolans Bore rare earth element (REE) deposit consists of a network of fluorapatite-bearing veins and breccias hosted within Proterozoic granulites of the Reynolds Range, Central Australia. Mineralisation is divided into three zones (north, central, and south-east), with the north and south-east zones consisting of massive REE-bearing fluorapatite veins, with minor brecciation and carbonate infill. The central zone is distinctively different in mineralogy and structure; it features extensive brecciation, a high allanite content, and a large, epidote-rich enveloping alteration zone. The central zone is a reworking of the original solid apatite veins that formed during the Chewings Orogeny at ca. 1525 Ma. These original apatite veins are thought to derive from phosphate-rich magmatic-hydrothermal fluid exsolved from as-yet unrecognised alkaline magmatic bodies at depth. We define four ore breccia types (BX1-4) in the central zone on the basis of detailed petrological and geochemical analysis of drillcore and thin sections. BX1 ore comprises fluorapatite with minor crackle brecciation with carbonate infill and resembles ore of the north and south-east zones. Breccia types BX2, BX3, and BX4 represent progressive stages of ore brecciation and development of calc-silicate mineral (amphibole, epidote, allanite, calcite) infill. Comparison of bulk ore sample geochemistry between breccia types indicates that REEs were not mobilised more than a few centimetres during hydrothermal alteration and brecciation. Instead, most of the REEs were partitioned from the original REE fluorapatite into newly formed allanite, REE-poor fluorapatite and minor REE carbonate in the breccias. Negative europium (Eu) anomalies in the breccia minerals are accounted for by a large positive Eu anomaly in epidote from the alteration zones surrounding the ore breccias. This observation provides a direct link between ore recrystallisation and brecciation, and the formation of the alteration halo in the surrounding host rocks. Where allanite and fluorapatite are texturally related, the fluorapatite is relatively depleted in the light rare earth elements (LREEs), whereas allanite is relatively LREE enriched, suggesting co-crystallisation. We tentatively date the BX1 ore stage to 1440 ± 80 Ma based on U-Pb dating of thorianite. Sm-Nd isotope isochrons derived from in situ isotope analysis of cognate apatite and allanite date the BX2 and BX3 events to ca. 400 Ma, while U-Pb dating of late-stage monazite from the BX4 ore stage returned an age of ca. 350 Ma. Therefore, formation of the central zone at Nolans Bore involved multiple alteration/brecciation events that collectively span over 1 billion years in duration. We suggest that the BX1-type veins and breccias were formed from REE-rich, saline (F- and Cl-bearing) fluids that infiltrated the granulite-grade host rocks in association with either shear activation events of the Redbank Shear Zone (1500-1400 Ma) or intrusion of late-stage pegmatites of the Mt Boothby area. BX2, BX3, and BX4 events record deformation and hydrothermal alteration associated with the Alice Springs Orogeny (400-350 Ma). These hydrothermal events occurred at temperatures of 450 to ~600 °C, due to inflow of highly acidic hydrous fluids derived from a magmatic source, or from mixing of meteoric and metamorphic fluids. Our data testify to the long and complex geological history of not only the Nolans Bore REE deposit, but also of the rocks of the eastern Reynolds Range, and demonstrate the great utility of using hydrothermally derived REE minerals to trace the timing of crustal deformation events and source of associated hydrothermal fluids.
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NASA Astrophysics Data System (ADS)
Sorokhtina, Natalia; Belyatsky, Boris; Antonov, Anton; Kononkova, Natalia; Lepekhina, Elena; Kogarko, Lia
2017-04-01
The alkaline-ultrabasic Gremyakha-Vyrmes massif occurs within the Central Kola terrane in the northern part of the Fennoscandian Shield and consists of diverse rock complexes: basic-ultrabasic rocks, foidolites, alkaline metasomatic rocks and carbonatites, alkaline granites and granosyenites. Nb-Zr ore deposit is confined to alkaline metasomatic rocks developed over foidolites. The metasomatites are represented by albitites and aegirinites occur as submeridionally orientated zones extending up to 6-8 km and several hundred meters thickness. They are mainly composed of albite and aegirine, but amphibole, annite, microcline, fluorapatite, titanite, ilmenite, pyrochlore group minerals, zircon are present [Sorokhtina et al., 2016]. Carbonatites are developed sporadically and accessory zircon but not the pyrochlore is observed only in contact zones with albitites and aegerinites. In metasomatites, zircon and pyrochlore are main rare metal minerals, which are formed at the latest stages of crystallization. Ca-dominant fluorcalcio- and hydroxycalciopyrochlores are the most abundant, whereas U-dominant pyrochlore, oxyuranobetafite, zero-valent-dominant (Ba, Sr-dominant) pyrochlore, hydro- or kenopyrochlore are rare. The pyrochlore-group minerals form heterogeneous metacrystals containing inclusions of host rock minerals, calcite, ilmenite, zircon, sulfides, and graphite. While pyrochlore is replaced by Si-rich "pyrochlore" (SiO2 is up to 18 wt.%.), cation-deficient hydrated pyrochlore, Fe-Si-Nb, U-Si-Nb, and Al-Si-Nb phases along fracture zones and margins. The early generation zircon is represented by large heterogeneous metacrystals filled with inclusions of various host rock minerals, calcite, ilmenite, thorite, thorianite and sulfides, while the late zircons are empty of inclusions. Zircons are nearly stoichiometric in composition; but intermediate zones are enriched in Pb, Y and Th, and overgrowths are enriched Hf only. According to CL and ion-microprobe analysis zircon has polygenetic nature: some relics inherited from foidolite crystallized at about 800°C, whereas the newly formed - at 600°C [Watson et al., 2006]. The time interval of the magmatic massif formation may be estimated as long as 80-100 Ma only. The basic-ultrabasic rocks and foidolites were intruded consistently at 1982 ± 6 Ma and 1894±12 according to SHRIMP-II U-Pb zircon dating, but the whole-rock Sm-Nd isotope dating has resulted in 1879±99 Ma and reflects the impact of alkaline granite intrusion (1871±9 Ma). The late differentiates from alkaline magma crystallization were the main source of rare metals for zircon-pyrochlore ores of alkaline metasomatites. The metasomatic rocks (aegirinites, albitites) and carbonatites were formed as late as 1910 ± 15 Ma (SHRIMP-II U-Pb zircon, titanite, pyrochlore). While some pyrochlore grains from metasomatites are showed that U-Pb age of ore formation is 1766 ± 24 and 1764 ± 19 respectively. That can be attributed to additional source of rare metals connected with fluids formed during regional metamorphism 1750 m.y. ago [Glebovitskii et al., 2014]. The last probable source of rare-metal material and ore-deposit evolution stage (recrystallization) is established by individual pyrochlore grain Sm-Nd and U-Pb systems and evidences tectono-thermal activity at the Paleozoic plume magmatism, which was followed by structural and chemical mineral changes. The research was done within the framework of the scientific program of Russian Academy of Sciences and state contract K41.2014.014 with Sevzapnedra. References: Watson E. B., Wark D. A., Thomas J. B. Crystallization thermometers for zircon and rutile // Contrib. Mineral. Petrol. 2006. 151, 413-433. Glebovitskii V.A., Bushmin S.A., Belyatsky B.V., Bogomolov E.S., Borozdin A.P., Savva E.V., Lebedeva Y.M. Rb-Sr age of metasomatism and ore formation in the low-temperature shear zones of the Fenno-Karelian craton, Baltic Shield // Petrology. 2014. 22(2). 184-204. Sorokhtina N.V., Kogarko L.N., Shpachenko A.K., Senin V. G. Composition and Conditions of Crystallization of zircon from the rare-metal ores of the Gremyakha-Vyrmes massif, Kola Peninsula // Geochemistry International. 2016. 54 (12). 1035-1048.
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Timing of metasomatism in a subcontinental mantle: evidence from zircon at Finero (Italy)
NASA Astrophysics Data System (ADS)
Badanina, I. Yu.; Malitch, K. N.
2012-04-01
The Finero phlogopite-peridotite represents a metasomatized residual mantle harzburgite, exposed at the base of the lower-crust section in the Ivrea Zone, Western Alps (Hartmann and Wedepohl 1993). It forms the core of a concentrically zoned sequence of internal layered gabbro, amphibole-rich peridotite and external gabbro. The phlogopite peridotite contains small-size chromitite bodies, with a suite of accessory minerals such as phlogopite, apatite, Ca-Mg carbonates, zirconolite, zircon, thorianite and uraninite, proposed to form during alkaline-carbonatitic metasomatism process within the mantle (Zaccarini et al. 2004). In this study, the combined application of a non-destructive technique to separate zircon from their host rocks (see details at http://www.natires.com) and in-situ analytical technique for compositional and isotopic analysis (SHRIMP-II at Russian Geological Research Institute, St. Petersburg) has provided new more detailed age constraints on the formation of chromitite and related metasomatic events within a mantle tectonite at Finero. Chromitite samples derived from the dump in the prospecting trenches of Rio Creves. In thin sections, zircon occurs as relatively large (up to 200 μm) grains characterized by subhedral to euhedral shapes. Separated grains of zircon form two distinct populations. Dominant zircon population is pale pink and characterized by different shapes (subhedral, subrounded or elongated). In cathodoluminescense, the main set of population is represented by complex grains, which show development of core-rim relationship (most likely recrystallized rim on a preserved core). Subordinate zircon grains are colourless. They are characterized by a smoky cathodoluminescense, with almost no internal pattern. Three main U-Pb age clusters have been recognized. The youngest age cluster, typical for subordinate colourless zircon population and rims in complex grains of dominant pale pink population, show two concordant 206Pb/238U ages (e.g., 208.6 ± 4.0 Ma, MSWD=2.0; P=0.16, n=8 and 194.9 ± 3.4 Ma, MSWD=0.45; P=0.50, n=3, respectively). Other age clusters are characterized by the cores and rims observed in composite grains. They yielded concordant 206Pb/238U ages of 288.3 ± 7.3 Ma (MSWD=3.3, n=6) and 248.6 ± 3.3 Ma (MSWD=0.13, P=0.72, n=8), respectively. Since the pioneering work of Exley et al. (1982), the complex metasomatic history at Finero has received much attention. New U-Pb results are consistent with the age range obtained for mantle rocks, the phlogopite peridotite (293 ± 13 Ma, Voshage et al. 1987) and chromitite (208 ± 2 Ma, Grieco et al. 2001). The former age estimate, based on a Rb-Sr whole-rock isochron for six phlogopite-bearing peridotites and one phlogopite pyroxenite, has been interpreted as time of K metasomatic enrichment of the harzburgite. This event has been coeval with the intrusion of alkaline ultramafic magmas into the deep crust of the Ivrea Zone during the late Carboniferous (287 ± 3 Ma, Garuti et al. 2001). The U-Pb age of 208±2 Ma for zircon at Alpe Polunia, attributed by Grieco et al. (2001) to one of the major metasomatic episodes, is corroborated by a subordinate subset of zircon grains at Rio Creves. The U-Pb zircon ages identified in this study thus show notable differences. Our U-Pb data do not concur with the assumption of a single metasomatic event during chromitite formation. In contrast, we suggest a prolonged formation and multistage evolution of zircon growth, as mirrored by multiple U-Pb ages. U-Pb results for zircons from two chromitite localities (Alpe Polunia and Rio Creves) place tight constraints on their different temporal evolution. We presume that Hf-isotope data of zircon and Os-isotope data of laurite, to be investigated in the future, will shed new light on the sources of materials involved in a subcontinental mantle at Finero. This investigation was supported by Uralian Branch of Russian Academy of Sciences (grant 12-P-5-1020).